Semicrystalline polymers toughening

Semicrystalline polyamide fine powders have been used as toughening agents for epoxy networks. The powders can be obtained by grinding granules, or directly by anionic polymerization of lactams, 6 or 12, in an organic solvent from which the formed semicrystalline polymer precipitates. Microporous powders with an average particle size in the range of 10 pm and a narrow particle-size distribution, are obtained. 

As PET has the ability to be obtained either in the amorphous state or not an additional scientific goal exists that is studying the interrelations between the microstructure of semicrystalline polymers and the efficiency of rubber toughening [ 1 ]. 

The addition of a second non-crystallizable component to a crystallizable matrix can cause drastic variations of important morphological and structural parameters of the semicrystalline phase, such as the shape, size, regularity of sphemlites and intersphemlitic boundary regions, lateral dimensions of the lamellae, etc. These factors may greatly influence the mechanical behavior and, in particular, the fracture mechanisms, and thus are of great importance, especially when the toughening of semicrystalline polymer blends is considered. 

Similarly the disadvantages of PC are the stress cracking and chemical sensitivity. Stress cracking can be treated as a part of impact properties and a simple solution may thus be addition of ABS or ASA. On the other hand, to improve the solvent resistance—a property that is particularly important in automobile applications—a semicrystalline polymer may be added. From Table 4.37, it is apparent that TPEs (e.g., PBT, PET) could provide that property, but they also lack warp resistance and impact strength. Hence an ideal blend for automobile application based on PC and TPEs should be impact modified with, for example, an acrylic latex copolymer. A schematic of preparation of this type of toughened blend introduced by GEC-Europe in 1979 under the tradename Xenoy is shown in Figure 4.41. 

The phenomenon is similar to the toughening effect in semicrystalline polymers caused by the strengthening of the amorphous regions with crystalline cross-links but in the latter case, the two-phase aspect arises from the existence of crystalline and amorphous regions. 

OlWan Wang, H.-H., Chen, J.-C. Toughening of epoxy resin by functional-terminated polyurethanes and/or semicrystalline polymer powders. J. Appl. Polym. Sci. 82 (2001) 2903-2912. 

The core of the book is devoted to subjects starting with anelastic behavior of polymers and rubber elasticity, but proceeds with greater emphasis in following chapters to mechanisms of plastic relaxations in glassy polymers and semicrystalline polymers with initial spherulitic morphology. Other chapters concentrate on craze plasticity in homo-polymers and block copolymers, culminating with a chapter on toughening mechanisms in brittle polymers. To make the... 

The dominant mechanism of deformation depends mainly on the type and properties of the matrix polymer, but can vary also with the test temperature, the strain rate, and the morphology, shape, and size of the modifier particles (Bucknall 1977, 1997, 2000 Michler 2005 Michler and Balta-Calleja 2012 Michler and Starke 1996). Properties of the matrix determine not only the type of the local yield zones but also the critical parameters for toughening. In amorphous polymers with the dominant formation of crazes, the particle diameter, D, is of primary importance, while in some other amorphous and in semicrystalline polymers with the dominant formation of dilatational shear bands or intense shear yielding, the interparticle distance ID, i.e., the thickness of the matrix ligaments between particles, seems to be also an important parameter influencing the efficiency of toughening. This parameter can be adjusted by various combinations of modifier particle volume fraction and particle size. 

Toughened Polymers with Semicrystalline Matrix. It is well known that toughness of semicrystalline polymers such as PA (polyamide) and PP can be increased similar to the amorphous poljuners by the addition of relatively small amounts of rubber particles such as EPR or EPDM. As in HIPS and ABS, the modifier particles act as stress concentrators, initiating a plastic deformation of matrix strands between the particles as the main energy absorption step. In impact-modified PA and PP at room temperature, plastic deformation takes place through shear deformation (mechanism of multiple shear deformation). 

The fracture of most semicrystalline polymers, notably polyethylene, polypropylene and nylon, cannot be described by LEFM based on the theory of Griffith and Irwin because large-scale yielding occurs at the crack tip prior to failure. For these materials and for toughened polymers and polymer blends other approaches have been developed, three of which will be discussed in detail ... 

Toughening of a semicrystalline polymer by a phase segregated block copolymer introduces several levels of complexity, as shown in an example of a nylon 66 toughened with a Kraton... 

Rubber-toughened polymers with an amorphous matrix and semicrystalline polymer matrix... 

The effect of matrix strand yielding occurs only below a critical interparticle distance which is dependent on the matrix polymer and the temperature and is similar to the concepts of critical interparticle distance and critical thickness seen in rubber-toughened semicrystalline polymers see Chapter 5. Small interparticle distances below the critical size and, therefore, optimum conditions for good toughness are easier to realize with small particles, in particular with nanoparticles [6]. 

Karger-Kocsis J (1996) How does phase traiisfonnation toughening work in semicrystalline polymers, Polym Eng Sci 36 203-210. 

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